Communication apparatus, time synchronization system, and time synchronization method

ABSTRACT

A communication apparatus ( 13 ) according to the present invention is provided with: a storage unit ( 131 ) that stored in advance difference information ( 1311 ), which is the difference between first time information (t 1 ) and second time information (t 2 ), said first time information (t 1 ) being received from a synchronization source communication apparatus ( 11 ) as a time synchronization source connected to a transmission system ( 12 ) wherein transmission delay time is different depending on the direction of transmission, and said second time information (t 2 ) obtained from a time synchronization origin ( 14 ) other than the transmission system ( 12 ); and a time synchronization means ( 132 ) that carries out, when the second time information (t 2 ) cannot be obtained from the time synchronization origin ( 14 ), time synchronization with the synchronization source communication apparatus ( 11 ) using the difference information ( 1311 ) read out from the storage unit ( 131 ).

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a National Stage Entry of International ApplicationNo. PCT/JP2013/002214, filed Apr. 1, 2013, which claims priority fromJapanese Patent Application No. 2012-166669, filed Jul. 27, 2012. Theentire contents of the above-referenced applications are expresslyincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a communication apparatus, a timesynchronization system, and a time synchronization method, particularlya communication apparatus, a time synchronization system, and a timesynchronization method for time synchronization in a transmission systemhaving asymmetrical delay characteristics in which transmission delaytime is different depending on the direction of transmission.

BACKGROUND ART

Generally, in a base station of a mobile communication, accuracy of timesynchronization of several micro seconds or less is required. A timesynchronization system, like GNSS (Global Navigation Satellite System),is therefore used. IEEE (The Institute of Electrical and ElectronicsEngineers) 1588 which defines the method for synchronizing a device on anetwork with accuracy of less than a micro second, can also be used fortime synchronization.

PTL 1 discloses a technology on the time synchronization network and thecommunication apparatus which achieve time synchronization withinrequired accuracy without increasing a network load or withoutincreasing cost of mounting a highly accurate oscillator, when a basestation which carries out time synchronization on the basis of GPS(Global Positioning System) and a base station which carries out timesynchronization on the basis of GPS through a packet network usingprotocol, like IEEE 1588 exist in the same network.

PTL 2 discloses the technology related to the time synchronizationsystem which synchronizes a time at a slave node to a time at a masternode using protocol, like IEEE 1588.

CITATION LIST Patent Literature

PTL 1: Japanese Patent Application Laid-Open No. 2010-278546

PTL 2: Japanese Patent Application Laid-Open No. 2011-135482

SUMMARY OF INVENTION Technical Problem

However, PTLs 1 and 2 include a problem in which time synchronizationbecome indefinite when a transmission path has asymmetrical delaycharacteristics. The reason is described below.

In order to stably carry out time synchronization in GNSS, sky view hasto be sufficiently secured. However, since full sky view may not beobtained in an urban area due to building wall placement or surroundingobstacles, stability problem exists. Using a broadcasting wave from asatellite, GNSS is a system which is not relatively subjected todisturbance from the outside. Holdover situation however may occur andtime synchronization may be lost because of suspension of GNSS,shielding of a transmission path, interference by an interrupting wave,failure of a receiving apparatus, or the like. Hence, fault-tolerancehas to be improved.

Therefore, a GNSS receiver is required to keep time synchronization byitself even in Holdover situation. In order to keep time synchronizationfor a practical-level period of time as described above, a large andexpensive frequency source, such as OCXO (Oven Controlled XtalOscillator) with high long-term stability, an atomic oscillator, or thelike, has to be used.

Accordingly in order to reduce cost of GNSS, as disclosed in PTL 1 or 2,IEEE 1855 system, like NTP (Network Time Protocol) or PTP (PrecisionTime Protocol), which carries out time synchronization by superimposingon a transmission system, is used. Since the above system issuperimposed on the transmission system, fluctuation of delay amountbetween transmission apparatuses may vary depending on transmission pathenvironment or delay amount may be different depending on a transmissionpath. Therefore, time synchronization becomes indefinite.

Here, the fluctuation of delay amount can be corrected to some extent byaveraging arrival times or pulling the shortest arrival. However, timesynchronization between apparatuses cannot be carried out as describedbelow, if delays of bi-directional transmission paths are different.

A situation in which a time of a slave device is synchronized with atime of a master device is explained below. Initially the master devicetransmits a packet including information of a time A1 in the masterdevice to the slave device. The time A1 is, for example, a transmittingtime of the packet. The slave device which has received the packet cancalculate its own time B1 by using a transmission delay time X from themaster device to the slave device on the basis of the following equation(1).B1=A1+X  (1)

At this time, however, since the times of the master device and theslave device do not synchronize, the transmission delay time X cannot becalculated accurately. Thus, a packet transmission delay time Y of theopposite direction is further used, that is, when the packet istransmitted from the slave device to the master device. That is to say,packets having information on the transmitting time are sent back byreturn between the master device and the slave device whose times aredetermined. Thereby the bidirectional delay amount is calculated and ahalf thereof is estimated as the transmission delay time X′ (e.g.equation (2)). The time B of the slave device is calculated on the basisof the following equation (3) and synchronization with the time A of themaster device becomes possible.X′=(X+Y)/2  (2)B=A+X′  (3)

Here, in the above description, delay time X=delay time Y is premised.If the delays of bi-directionally transmission paths are different, timedifference between X and Y becomes an offset error of the time B andautonomous resolution is impossible. Therefore, time synchronizationbetween the master device and the slave device cannot be carried out andtime synchronization becomes indefinite.

The invention is made in consideration of the above problem, and anobject thereof is to provide a communication apparatus, a timesynchronization system, and a time synchronization method which keeptime synchronization accuracy in a transmission system havingasymmetrical delay characteristics in which a transmission delay time isdifferent depending on the direction of transmission.

Solution to Problem

A communication apparatus of a first aspect of the invention includes astorage unit that stores in advance difference information between firsttime information and second time information, the first time informationbeing received from a synchronization source communication apparatus, asa time synchronization source, connected to a transmission systemwherein a transmission delay time is different depending on thedirection of transmission and the second time information obtained froma time synchronization origin other than the transmission system, andtime synchronization means for carrying out, when the second timeinformation cannot be obtained from the time synchronization origin,time synchronization with the synchronization source communicationapparatus using the difference information read out from the storageunit.

A time synchronization system of a second aspect of the inventionincludes a transmission system in which a transmission delay time isdifferent depending on the direction of transmission, a firstcommunication apparatus that is connected to the transmission system, asa time synchronization source, a time synchronization origin other thanthe transmission system, and a second communication apparatus that isconnected to the transmission system and the time synchronization originand carries out time synchronization on the basis of time information ofthe first communication apparatus. The second communication apparatusincludes a storage unit that stores in advance difference informationbetween first time information received from the first communicationapparatus and second time information obtained from the timesynchronization origin, and time synchronization means for carrying out,when the second time information cannot be obtained from the timesynchronization origin, time synchronization with the firstcommunication apparatus using the difference information read out fromthe storage unit.

A time synchronization method of a third aspect of the inventionincludes receiving first time information from a synchronization sourcecommunication apparatus, as a time synchronization source, connected toa transmission system wherein transmission delay time is differentdepending on the direction of transmission, obtaining second timeinformation from a time synchronization origin other than thetransmission system, calculating difference information between thefirst time information and the second time information, storing thedifference information in a storage unit, after that, reading out thedifference information from the storage means when the second timeinformation cannot be obtained from the time synchronization origin, andcarrying out time synchronization with the synchronization sourcecommunication apparatus using the difference information read out fromthe storage unit.

Advantageous Effects of Invention

The invention is made in consideration of the above problems, and canprovide a communication apparatus, a time synchronization system, and atime synchronization method which keep time synchronization accuracy ina transmission system having asymmetrical delay characteristics in whicha transmission delay time is different depending on the direction oftransmission.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a timesynchronization system of an exemplary embodiment 1 of the invention.

FIG. 2 is a block diagram illustrating a configuration of a timesynchronization system of an exemplary embodiment 2 of the invention.

FIG. 3 is a block diagram illustrating a configuration of a slave deviceof the exemplary embodiment 2 of the invention.

FIG. 4 is a flowchart explaining a flow of a time synchronizationprocess of the exemplary embodiment 2 of the invention.

FIG. 5 is a block diagram illustrating a configuration of a slave deviceof an exemplary embodiment 3 of the invention.

DESCRIPTION OF EMBODIMENTS

Specific exemplary embodiments to which the invention is applied aredescribed below in detail by referring to drawings. In each of drawings,the same element has the same sign, and repeated explanations areomitted as necessary in order to clarify the explanations.

An Exemplary Embodiment 1 of the Invention

FIG. 1 is a block diagram illustrating a configuration of a timesynchronization system 100 of an exemplary embodiment 1 of theinvention. The time synchronization system 100 includes asynchronization source communication apparatus 11, a transmission system12, a communication apparatus 13, and a time synchronization origin 14.In the transmission system 12, a transmission delay time is differentdepending on the direction of transmission. That is to say, thetransmission system 12 includes asymmetrical transmission delaycharacteristics. The synchronization source communication apparatus 11is connected to the transmission system 12 and is a first communicationapparatus as a synchronization source of a time. The synchronizationsource communication apparatus 11 is, for example, a master device intime synchronization. The time synchronization origin 14 is acommunication apparatus, a transmission system, a satellite positioningsystem, satellite navigation system, or the like, which is a timesynchronization origin other than the transmission system 12.

The communication apparatus 13 is a second communication apparatus whichis connected to the transmission system 12 and the time synchronizationorigin 14 and carries out time synchronization on the basis of firsttime information t1 of the synchronization source communicationapparatus 11. The communication apparatus 13 is a slave device in timesynchronization.

The communication apparatus 13 includes a storage unit 131 and a timesynchronization unit 132. The storage unit 131 stores in advancedifference information 1311 between the first time information t1received from the synchronization source communication apparatus 11 andsecond time information t2 obtained from the time synchronization origin14. If the time synchronization unit 132 cannot obtain the second timeinformation t2 from the time synchronization origin 14, the unit 132carries out time synchronization with the synchronization sourcecommunication apparatus 11 using the difference information 1311 readout from the storage unit 131.

Carrying out time synchronization with the synchronization sourcecommunication apparatus 11 via the transmission system 12 having anasymmetrical transmission delay time, the time synchronous system 100and communication apparatus 13 of the exemplary embodiment 1 of theinvention use the second time information t2 obtained from the timesynchronization origin 14 which gives a stable time. By storing inadvance the difference information 1311 between the first timeinformation t1 and the second time information t2, it is also possibleto carry out time synchronization using the difference information 1311which is already stored even though the latest second time informationt2 cannot be obtained from the time synchronization origin 14.Therefore, in a transmission system having asymmetrical delaycharacteristics in which a transmission delay time is differentdepending on the direction of transmission, accuracy of timesynchronization can be maintained.

An Exemplary Embodiment 2 of the Invention

FIG. 2 is a block diagram illustrating a configuration of a timesynchronization system 200 of an exemplary embodiment 2 of theinvention. The time synchronization system 200 is one example of theexemplary embodiment 1. The time synchronization system 200 includes aGNSS 21, a master device 22, a transmission system 23, a GNSS 24, and aslave device 25. The time synchronization system 200 synchronizes timeinformation 250 of the slave device 25 with time information 220 of themaster device 22.

The GNSS 21 and the GNSS 24 are a satellite positioning systems orsatellite navigation systems which are different from each other. TheGNSS 21 includes a GNSS satellite 211 and a GNSS receiver 212. The GNSSreceiver 212 receives a signal from the GNSS satellite 211 and transmitstime/timing pulse output p1. The GNSS 24 includes a GNSS satellite 241and a GNSS receiver 242. The GNSS receiver 242 receives a signal fromthe GNSS satellite 241 and transmits time/timing pulse output p2. TheGNSS 24 is an example of the time synchronization origin 14 abovedescribed.

The GNSS 21 includes fault-tolerance such as redundancy. On the otherhand, in the GNSS 24, redundancy and fault tolerance for low cost andminiaturization are in a low level, compared with the GNSS 21. Since theGNSS 21 and the GNSS 24 can employ publicly known technologies if theycan be time reference, detailed explanations thereof are omitted. As theGNSS 21 and the GNSS 24, a time synchronization system, like GPS,Galileo (Galileo (positioning system)), GLONASS (Global NavigationSatellite System), or Beidou (BeiDou Navigation Satellite System) can beemployed.

The master device 22 is one example of the synchronization sourcecommunication apparatus 11 above described. The master device 22acquires the time/timing pulse output p1 which is time reference fromthe GNSS receiver 212, and holds as the time information 220. The masterdevice 22 is connected to the transmission system 23 in order totransmit and receive a packet for time synchronization with the slavedevice 25 controlled thereby. Since the other configurations in themaster device 22 can use publicly known technologies, detailedexplanations thereof are omitted.

The transmission system 23 is one example of the transmission system 12described above. The transmission system 23 includes communicationrelaying apparatuses 231 to 233. A transmission path from the masterdevice 22 to the slave device 25 is routed through the communicationrelaying apparatus 231. On the other hand, a transmission path from theslave device 25 to the master device 22 is routed through thecommunication relaying apparatuses 233 and 232. That is to say, in thetransmission system 23, bidirectional transmission paths between themaster device 22 and the slave device 25 are different each other. Inthe transmission system 23, therefore, bidirectional transmission delaytimes are different each other, that is, asymmetrical delaycharacteristics exist. FIG. 2 shows a case in which, in the transmissionsystem 23, the bidirectional transmission paths are different from eachother. Specifically, a case is shown, in which the number of thecommunication relaying apparatuses which is located between the masterdevice 22 and the slave device 25 is different depending on thedirection of transmission. However, a route of the transmission system23 is not limited thereto. For example, like ADSL (Asymmetric DigitalSubscriber Line), one which includes asymmetrical delay characteristicsin which upstream communication speed is different from downstreamcommunication speed in the same bidirectional transmission path may beemployed. Since publicly known technologies are employed for thetransmission system 23 and the communication relaying apparatuses 231 to233, detailed explanations thereof are omitted. As the transmissionsystem 23, a bidirectional network system, like Ethernet (trade mark),can be used.

The slave device 25 is one example of the communication apparatus 13above described. The slave device 25 is connected to the transmissionsystem 23 and receives the time information from the slave device 25.Further, the slave device 25 acquires, separately from the transmissionsystem 23, the time/timing pulse output p2 which is time reference fromthe GNSS receiver 242. Furthermore, the slave device 25 calculatesdifference information between the time information 220 and thetime/timing pulse output p2, and stores the difference information inthe storage unit. Then, the slave device 25 calculates and holds thetime information 250 on the basis of the time information 220 and thetime/timing pulse output p2. Besides, the slave device 25 outputs, as aninterface to transfer to a wireless base station, or the like (notshown), time/timing pulse output p3 regenerated from the master device22 or the GNSS 24.

Hereafter, when the slave device 25 cannot acquire the time/timing pulseoutput p2 from the GNSS 24, the slave device 25 reads out differenceinformation from the storage unit, carries out time synchronization withthe master device 22 using the read difference information, and outputsthe time/timing pulse output p3.

While synchronization protocol used by the master device 22 and theslave device 25 are not specified in the example above described, atime•timing transmission system such as IEEE 1588v2 or NTP protocol canbe used.

FIG. 3 is a block diagram illustrating a configuration of the slavedevice 25 of the exemplary embodiment 2 of the invention. Sinceconfigurations other than the slave device 25 in FIG. 3 are equivalentto FIG. 2, the explanations are omitted. The slave device 25 includes atiming correction information holding unit 251, a synchronizationprotocol termination unit 252 and a timing comparator 253.

The synchronization protocol termination unit 252 terminates a packetfor time synchronization from the master device 22, calculatestransmission delay times td1 and td2 between the master device 22 andthe slave device 25 and outputs the regenerated time/timing pulse outputp3. Besides, the synchronization protocol termination unit 252calculates a time in the slave device 25 in consideration of anasymmetrical transmission delay time of the transmission system 23.Publicly known technologies can be used for the synchronization protocoltermination unit 252.

The timing comparator 253 calculates time/timing difference informationd using the time information 250 of the slave device 25 calculatedwithout considering asymmetrical transmission delay time for eachtransmission path (time/timing pulse output p3) and a reference time onthe basis of the time/timing pulse output p2 acquired from the GNSS 24.Then, the synchronization protocol termination unit 252 calculates thetransmission delay time for each transmission path td1 and td2 andstores the result in the timing correction information holding unit 251.Publicly known technologies can be employed for the timing comparator253.

The timing correction information holding unit 251 is a storageapparatus which stores the transmission delay times td1 and td2. Forexample, the transmission delay time td1 is a delay time in thetransmission from the master device 22 to the slave device 25, and thetransmission delay time td2 is a delay time in the transmission fromslave device 25 to the master device 22. That is to say, thesynchronization protocol termination unit 252 stores the bidirectionaltransmission delay times between the slave device 25 and the masterdevice 22 into the timing correction information holding unit 251, asdifference information.

Here, the timing comparator 253 and the synchronization protocoltermination unit 252 may be difference information calculation meanswhich calculates the difference information (time/timing differenceinformation d) between the first time information (time information 220)and the second time information (time/timing pulse output p2), andstores the difference information into the timing correction informationholding unit 251. That is to say, the synchronization protocoltermination unit 252 may store the difference information itself intothe timing correction information holding unit 251.

When the time/timing pulse output p2 can be acquired from the GNSS 24,the synchronization protocol termination unit 252 carries out timesynchronization with the master device 22 on the basis of thetime/timing pulse outputs p1 and p2.

FIG. 4 is a flowchart explaining a flow of a time synchronizationprocess of the exemplary embodiment 2 of the invention. Initially, thesynchronization protocol termination unit 252 receives the first timeinformation (e.g. time information 220) from the master device 22 (S11).The timing comparator 253 tries to acquire the second time information(e.g. time/timing pulse output p2) from the GNSS receiver 242 which is atime synchronization origin (S12). Whether or not acquisition of thesecond time information from the GNSS receiver 242 becomes successful isdetermined at this point (S13).

If the GNSS 24 is normal, acquisition of the second time information issuccessful (YES in step S13), and the timing comparator 253 calculatesthe time/timing difference information d, as the difference information(S14). The synchronization protocol termination unit 252 stores thedifference information in the timing correction information holding unit251 (S15). In addition, the synchronization protocol termination unit252 carries out time synchronization with the master device 22 using thedifference information (S17).

On the other hand, when it is determined that acquisition of the secondtime information from the GNSS receiver 242 fails in step S13, thesynchronization protocol termination unit 252 reads out the differenceinformation from the timing correction information holding unit 251(S16). The case that the GNSS 24 fails and the case that the slavedevice 25 cannot fully receive the time/timing pulse outputs p2 due to aradio wave condition, etc. are exemplified.

The synchronization protocol termination unit 252 carries out timesynchronization with the master device 22 using the read differenceinformation (S17).

Subsequently, calculation of the transmission delay time in the slavedevice 25 of the exemplary embodiment 2 is described below. Thetransmission delay time td1 is a time which is required for transmissionof a specific packet from the master device 22 to the slave device 25.Here, suppose a transmitting time of the packet in the master device 22is “A1”.

The synchronization protocol termination unit 252 receives the packetincluding the transmitting time A1 from the master device 22. Next, thesynchronization protocol termination unit 252 regenerates thetime/timing pulse outputs p3 based on the transmitting time A1. Besides,the slave device 25 acquires the reference time B1 based on thetime/timing pulse outputs p2 from the GNSS receiver 242. Then, thetiming comparator 253 can calculate the transmission delay time td1 asthe time/timing difference information d by comparing the transmittingtime A1 with the reference time B1. Here, the transmission delay timetd1 can be calculated using the following equation (4).td1=B1−A1  (4)

The slave device 25 in the exemplary embodiment 2 stores thetransmission delay time td1 into the timing correction informationholding unit 251. Therefore, even though the time reference based on theGNSS 24 is lost, the slave device 25 can calculate a time B1′ of theslave device 25 from the transmitting time A1 using the transmissiondelay time td1 which is already held in the timing correctioninformation holding unit 251. Therefore, it is possible to keep timesynchronization. Here, the time B1′ of the slave device 25 can becalculated by the following equation (5).B1′=A1+td1  (5)

Besides, the transmission delay time td2 is a time which is required fortransmission of a specific packet from the slave device 25 to the masterdevice 22. Here, suppose a transmitting time of the packet in the slavedevice 25 is “B2”.

Initially, the slave device 25 transmits the packet including thetransmitting time B2 to the master device 22. Next, by comparing a timeA2 at which the packet is received with the transmitting time B2included in the packet, the master device 22 can calculate thetransmission delay time td2. These techniques can be achieved on thebasis of IEEE 1588. The transmission delay time td2 can be calculated bythe following equation (6).td2=A2−B2  (6)

After that, the master device 22 informs the slave device 25 of thetransmission delay time td2. The slave device 25 stores the transmissiondelay time td2 into the timing correction information holding unit 251.Thereby appropriateness can be examined even though a transmission pathof the transmission system 23 is changed. The reason is explained below.

Initially, in the time synchronization system, transmission delay amountbetween the master device 22 and the slave device 25 can be calculatedon the basis of a round-trip time Z of a packet. In this case, when around-trip time of a packet for time synchronization is represented asZ, it can be shown as the following equation (7).Z=X+Y  (7)

For that reason, it is potentially considered that the time B2′ of theslave device 25 can be calculated without using the transmission delaytime td2 when the time reference based on the GNSS 24 is lost. However,the transmission delay amount changes if a transmission path in thetransmission system 23 is changed. In this case, the round-trip time Zalso changes. Therefore, accuracy of the time B2′ cannot be kept if Z isused. The slave device 25 cannot examine appropriateness of thetransmission delay amount based only on the transmission delay time td1.Therefore, as described above, the slave device 25 has to store thetransmission delay time td2 in the timing correction information holdingunit 251.

As described above, the time synchronization system 200 of the exemplaryembodiment 2 includes the following advantageous effects. Initially, inthe time synchronization system having asymmetrical delay in thetransmission path thereof, by measuring an asymmetrical delay time ofthe transmission path using time information as a reference, accuracy oftime synchronization can be kept even though a failure of the timeinformation as a reference occurs. Besides, even though a failure of thetime information which is a reference for GNSS occurs, since thereference time is kept in the time synchronization system, a low-costfrequency source can be used.

An Exemplary Embodiment 3 of the Invention

An exemplary embodiment 3 of the invention is a modified example of theabove embodiment 2. The difference from the exemplary embodiment 2 is touse the transmission system in which bidirectional transmission delaytimes are symmetrical as a synchronization origin of the slave deviceinstead of GNSS.

FIG. 5 is a block diagram illustrating a configuration of a slave device25 a of the exemplary embodiment 3 of the invention. The sameconfigurations as those of FIG. 3 are indicated with the same signs andexplanations thereon are omitted. Initially, a master device 27 is acommunication apparatus which is a synchronization source which isdifferent from the master device 22. The master device 27 is one exampleof the time synchronization origin 14 of FIG. 1. In a transmissionsystem 26, the bidirectional transmission delay times are symmetrical.

The slave device 25 a includes a synchronization protocol terminationunit 254 and a timing correction information holding unit 255 inaddition to the timing correction information holding unit 251, thesynchronization protocol termination unit 252, and the timing comparator253. The synchronization protocol termination unit 254 terminates apacket for time synchronization from the master device 27, calculatestransmission delay times td3 and td4 between the master device 27 andthe slave device 25, and outputs regenerated time/timing pulse. Further,the synchronization protocol termination unit 254 stores thetransmission delay times td3 and td4 in the timing correctioninformation holding unit 255. Since the transmission system 26 includesthe symmetrical transmission delay times, the synchronization protocoltermination unit 254 can calculate the transmission delay times td3 andtd4 by using existing techniques. The timing correction informationholding unit 255 is equivalent to the timing correction informationholding unit 251.

For this reason, even if the transmission system 23 includesasymmetrical delay characteristics, the slave device 25 a can stablyacquire the reference time from the transmission system 26. Inparticular, even if the reference time is not acquired from the masterdevice 27, by using the transmission delay times td3 and td4 stored inadvance in the timing correction information holding unit 255, it ispossible to give redundancy to the slave device 25 a.

Even though the transmission system 26 includes asymmetrical delaycharacteristics in FIG. 5, by holding in advance the transmission delaytimes td3 and td4 in the timing correction information holding unit 255,equivalent redundancy can be kept.

Other Description of Exemplary Embodiment

A conventional time synchronization system has the following problems. Afirst problem is that correct time synchronization cannot be calculateddue to a state of asymmetrical delay of a network communication path inthe time synchronization system. A second problem is that sincefault-tolerance of GNSS has to be increased when correct timesynchronization cannot be calculated, a time synchronization error hasto be minimized by using an expensive frequency source with highaccuracy.

Therefore, the exemplary embodiment 1 and the like of the inventiondescribed above improve the problem that accuracy of synchronizationreference cannot be acquired due to the transmission asymmetry of thetime synchronization system, and provide a means of solving using thetime synchronization system for improvement of fault-tolerance of GNSS.That is to say, in the time synchronization system in which thetransmission path has the asymmetrical delay time, another referencetime is set, the asymmetrical delay time of the transmission path ismeasured using the reference time to be used for calculation of the timesynchronization system. Thereby uncertainty of time synchronization inwhich the transmission path has asymmetrical delay characteristics canbe resolved in advance.

The slave device 25 in FIG. 3, in particular, includes the timingcorrection information holding unit 251. Further, the slave device 25calculates the time/timing difference information d of a regenerationtime of the master device 22 based on the reference time, using thetime/timing pulse output p2 of the GNSS 24 and the time/timing pulseoutput p3 regenerated from the time information received from the masterdevice 22. After that, the slave device 25 calculates in advanceasymmetrical transmission delay times td1 and td2 for each transmissionpath in the transmission system 23 by using the time/timing differenceinformation d.

By calculating in advance the asymmetrical transmission delay times foreach transmission path, it is resolved that uncertainty of timesynchronization occurs only by the time synchronization system when thetransmission delay time for each transmission path is different. Therebythe reference time can be kept only by the time synchronization systemeven when GNSS goes wrong.

The exemplary embodiment 1, etc, calculate a total of delay times ofrespective transmission paths in the time synchronization system usingthe time information from a time synchronization source of GNSS, etc.and the time information calculated in the time synchronization systemin which the transmission path includes asymmetrical delay. The timesynchronization system is operated using the total of the delay times inconsideration of asymmetry of each transmission path, so that accuracyof time synchronization in the time synchronization system havingasymmetrical delay in the transmission path is guaranteed.

As a specific example, in a situation in which reception accuracy isguaranteed by a GNSS receiver in the slave device, a time of anapparatus is determined using a timing pulse which is synchronized withGNSS and time information. On the other hand, the time synchronizationsystem is also operated and a synchronization operation with a differentapparatus synchronized with GNSS is carried out. Here, in order tocalculate a delay time for each transmission path, the time informationcalculated from the GNSS receiver, the synchronization time calculatedin the time synchronization system and the bidirectional delay time areused. Thus, when the time information cannot be acquired from GNSS, thetime synchronization system is used by using delay time information foreach transmission path which is calculated in a normal state. Therebytime synchronization accuracy can be kept.

The exemplary embodiments 1 to 3 can be applied to a wireless basestation apparatus which is required to accurately use the timesynchronization, such as 3G (3rd Generation), LTE (Long Term Evolution),WiMAX (Worldwide Interoperability for Microwave Access), or PHS(Personal Handy-phone System), and which is difficult to stably use theGNSS synchronization system.

The invention of the present application is not limited to the abovementioned exemplary embodiments. It is to be understood that to theconfigurations and details of the invention of the present application,various changes can be made within the scope of the invention of thepresent application.

A whole part or part of the exemplary embodiments disclosed above can bedescribed as, but not limited to, the following supplementary notes.

(Supplementary Note 1)

A communication apparatus which includes

a storage unit for storing in advance difference information betweenfirst time information and second time information, the first timeinformation being received from a synchronization source communicationapparatus as a time synchronization source connected to a transmissionsystem wherein transmission delay time is different depending on thedirection of transmission and the second time information obtained froma time synchronization origin other than the transmission system, andtime synchronization means for carrying out, when the second timeinformation cannot be obtained from the time synchronization origin,time synchronization with the synchronization source communicationapparatus using the difference information read out from the storageunit.(Supplementary Note 2)

The communication apparatus described in the supplementary note 1 whichis characterized in further including difference information calculationmeans for calculating the difference information between first timeinformation and second time information and for storing the differenceinformation in the storage unit.

(Supplementary Note 3)

The communication apparatus described in the supplementary note 2 whichis characterized in that the time synchronization means carries out timesynchronization with the synchronization source communication apparatuson the basis of the first time information and the second timeinformation when the second time information can be obtained from thetime synchronization origin.

(Supplementary Note 4)

The communication apparatus described in the supplementary note 2 whichis characterized in that the difference information calculation meansstores bidirectional transmission delay times of the means itself andthe synchronization source communication apparatus, as the differenceinformation, into the storage unit.

(Supplementary Note 5)

The communication apparatus of any one of the supplementary notes 1 to 4which is characterized in that the time synchronization origin is asatellite positioning system.

(Supplementary Note 6)

The communication apparatus of any one of the supplementary notes 1 to 4which is characterized in that the time synchronization origin is atransmission system in which bidirectional transmission delay times aresymmetrical.

(Supplementary Note 7)

A time synchronization system, including,

a transmission system in which transmission delay time is differentdepending on the direction of transmission,

a first communication apparatus that is connected to the transmissionsystem, as a time synchronization source,

a time synchronization origin other than the transmission system, and

a second communication apparatus that is connected to the transmissionsystem and the time synchronization origin and carries out timesynchronization on the basis of time information of the firstcommunication apparatus,

the second communication apparatus includes

a storage unit for storing in advance difference information betweenfirst time information received from the first communication apparatusand second time information obtained from the time synchronizationorigin, and

time synchronization means for carrying out, when the second timeinformation cannot be obtained from the time synchronization origin,time synchronization with the first communication apparatus using thedifference information read out from the storage unit.

(Supplementary Note 8)

The time synchronization system described in the supplementary note 7characterized in that the second communication apparatus furtherincludes difference information calculation means for calculating thedifference information between first time information and second timeinformation and for storing the difference information in the storageunit.

(Supplementary Note 9)

The time synchronization system described in the supplementary note 8characterized in that the time synchronization means carries out timesynchronization with the first communication apparatus on the basis ofthe first time information and the second time information when thesecond time information can be obtained from the time synchronizationorigin.

(Supplementary Note 10)

The time synchronization system described in the supplementary note 8characterized in that the difference information calculation meansstores bidirectional transmission delay times of the means itself andthe first communication apparatus, as the difference information, intothe storage unit.

(Supplementary Note 11)

A time synchronization method, including

receiving first time information from a synchronization sourcecommunication apparatus as a time synchronization source connected to atransmission system wherein transmission delay time is differentdepending on the direction of transmission,

obtaining second time information from a time synchronization originother than the transmission system,

calculating difference information between the first time informationand the second time information,

storing the difference information in a storage means,

after that, reading out the difference information from the storagemeans when the second time information cannot be obtained from the timesynchronization origin, and

carrying out time synchronization with the synchronization sourcecommunication apparatus using the difference information read out fromthe storage means.

The invention of the present application is explained by referring tothe exemplary embodiments. The invention of the present application isnot limited to the above description. It is to be understood that to theconfigurations and details of the invention of the present application,a person ordinarily skilled in the art can make various changes withinthe scope of the invention of the present application.

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-166669 filed on Jul. 27, 2012, thedisclosure of which is incorporated herein in its entirety by reference.

REFERENCE SIGNS LIST

-   100 time synchronization system-   11 synchronization source communication apparatus-   12 transmission system-   13 communication apparatus-   131 storage unit-   1311 difference information-   132 time synchronization unit-   14 time synchronization source-   t1 first time information-   t2 second time information-   200 time synchronization system-   21 GNSS-   211 GNSS satellite-   212 GNSS receiver-   22 master device-   220 time information-   23 transmission system-   231 communication relaying apparatus-   232 communication relaying apparatus-   233 communication relaying apparatus-   24 GNSS-   241 GNSS satellite-   242 GNSS receiver-   25 slave device-   25 a slave device-   250 time information-   251 timing correction information holding unit-   252 synchronization protocol termination unit-   253 timing comparator-   254 synchronization protocol termination unit-   255 timing correction information holding unit-   26 transmission system-   27 master device-   p1 time/timing pulse output-   p2 time/timing pulse output-   p3 time/timing pulse output-   d time/timing difference information-   td1 transmission delay time-   td2 transmission delay time-   td3 transmission delay time-   td4 transmission delay time

The invention claimed is:
 1. A communication device, comprising: amemory storing instructions; a storage unit configured to storedifference information reflecting a difference between first timeinformation and second time information a processor configured toexecute the instructions to: receive the first time information from afirst communication apparatus connected to a transmission system; obtainthe second time information from a time synchronization origin connectedto a second communication apparatus, wherein a first transmission delaytime in a first direction from the first communication apparatus to thesecond communication apparatus through a transmission system isdifferent than a second transmission delay time in a second directionfrom the second communication apparatus to the first communicationapparatus through the transmission system; determine whether the secondtime information can be obtained from the time synchronization origin;and implement, when it is determined that the second time informationcannot be obtained from the time synchronization origin, timesynchronization with the first communication apparatus using thedifference information read out from the storage unit.
 2. Thecommunication device of claim 1, wherein the processor is furtherconfigured to execute the instructions to: calculate the differenceinformation and to store the difference information to the storage unit.3. The communication device of claim 1, wherein the processor is furtherconfigured to execute the instructions to carry out time synchronizationwith the first communication apparatus on the basis of the first timeinformation and the second time information when it is determined thatthe second time information can be obtained from the timesynchronization origin.
 4. The communication device of claim 1, whereinthe first communication apparatus comprises a synchronization sourcecommunication apparatus configured to receive a time reference from asatellite positioning system.
 5. The communication device of claim 1,wherein the time synchronization origin is a satellite positioningsystem.
 6. The communication device of claim 1, wherein the timesynchronization origin is a second transmission system in whichbidirectional transmission delay times are symmetrical.
 7. Thecommunication device of claim 1, wherein the first communicationapparatus is a master device, and the second communication apparatus isa slave device.
 8. The communication device of claim 7, wherein themaster device is configured to transmit and receive a packet for timesynchronization with the slave device via the transmission system. 9.The communication device of claim 8, wherein the transmission systemcomprises a different quantity of communication relaying apparatusesalong a first transmission path in the first direction and a secondtransmission path in the second direction.
 10. The communication deviceof claim 1, wherein a first transmission speed in the first direction isdifferent from a second transmission speed in the second direction. 11.A time synchronization system, comprising: a transmission system inwhich a transmission delay time is different depending on a direction oftransmission; a first communication apparatus that is connected to thetransmission system and configured as a time synchronization source; atime synchronization origin; and a second communication apparatusconnected to the transmission system and the time synchronizationorigin, wherein the second communication apparatus comprises: a memorystoring instructions; a storage unit configured to store differenceinformation reflecting a difference between first time informationreceived from the first communication apparatus and second timeinformation obtained from the time synchronization origin, wherein afirst transmission delay time in a first direction from the firstcommunication apparatus to the second communication apparatus throughthe transmission system is different than a second transmission delaytime in a second direction from the second communication apparatus tothe first communication apparatus through the transmission system; and aprocessor configured to execute the instructions to: determine whetherthe second time information can be obtained from the timesynchronization origin; and implement, when it is determined that thesecond time information cannot be obtained from the time synchronizationorigin, time synchronization with the first communication apparatususing the difference information read out from the storage unit.
 12. Thetime synchronization system of claim 11, wherein the processor isfurther configured to execute the instructions to calculate thedifference information and to store the difference information in thestorage unit.
 13. The time synchronization system of claim 12, whereinthe processor is further configured to execute the instructions tostore, in the storage unit, bidirectional transmission delay times oftransmission between the second communication apparatus and the firstcommunication apparatus, as the difference information.
 14. The timesynchronization system of claim 11, wherein the processor is furtherconfigured to execute the instructions to implement time synchronizationwith the first communication apparatus on the basis of the first timeinformation and the second time information when it is determined thatthe second time information can be obtained from the timesynchronization origin.
 15. The time synchronization system of claim 11,wherein a first transmission speed in the first direction is differentfrom a second transmission speed in the second direction.
 16. The timesynchronization system of claim 11, wherein the transmission systemcomprises a different quantity of communication relaying apparatusesalong a first transmission path in the first direction and a secondtransmission path in the second direction.
 17. The time synchronizationsystem of claim 11, wherein the time synchronization origin comprises asatellite positioning system.
 18. A time synchronization method,comprising: receiving first time information from a first communicationapparatus connected to a transmission system; obtaining second timeinformation from a time synchronization origin connected to a secondcommunication apparatus, wherein a first transmission delay time in afirst direction from the first communication apparatus to the secondcommunication apparatus through the transmission system is differentthan a second transmission delay time in a second direction from thesecond communication apparatus to the first communication apparatusthrough the transmission system; calculating difference informationreflecting a difference between the first time information and thesecond time information; storing the difference information in a storageunit; determining whether the second time information can be obtainedfrom the time synchronization origin; and reading out the differenceinformation from the storage unit when it is determined that the secondtime information cannot be obtained from the time synchronizationorigin; and implementing time synchronization with the firstcommunication apparatus using the difference information read out fromthe storage unit.
 19. The time synchronization method of claim 18,comprising directing a first transmission path in the first directionthrough a different quantity of communication relaying apparatuses thana second transmission path in the second direction.
 20. The timesynchronization method of claim 18, wherein a first transmission speedin the first direction is different from a second transmission speed inthe second direction.